Psoriasis is one of inflammatory keratosis syndromes, and is a chronic intractable dermal disease characterized by scale. Psoriasis is a disease repeating remission and animus in many cases, and there is no decisive method for treating psoriasis to date.
Psoriasis is roughly classified into five types (1) psoriasis vulgaris, (2) erythroderma psoriaticum, (3) psoriasis arthropica, (4) psoriasis guttata, and (5) pustular psoriasis, based on the pathologic characteristics. Among them, psoriasis vulgaris, of which patients are largest in number, is a disease accounting for about 80% of all the patients with psoriasis, and pustular psoriasis is a disease accompanied with systemic symptom, leading to death in some cases. In any type, pathology of psoriasis is accompanied with both of immunological abnormality in epidermis and dermis, and abnormality in proliferation and differentiation of an epidermic keratinocyte.
Causes of onset for psoriasis are involved in both of genetic background and exogenous factor, and the psoriasis is developed by exposing any exogenous or endogenous factor to a person having a certain kind of genetic background.
It is well known that the genetic factor is related to the onset of psoriasis. In Europe and the U.S.A., a ratio of familial development thereof is about one-third, and an incidence rate of psoriasis in monozygotic twin shows 72%, which is high consistency to the others, and incidence rate of psoriasis in dizygotic twin is as low as 22%. As described above, in psoriasis, although it is certain that a genetic factor is an important cause of onset, but its hereditary manner is not clear. In addition, it is considered that there are little cases of onset caused by a single gene, so that onset is caused by multiple genes in many cases.
PSOR1 (Psoriasis susceptibility) gene localized in HLA antigen gene region of chromosome 6 is a psoriasis-sensitive gene which was happened to be found by typing of the HLA during the kidney transplantation. As genes involved in the onset of psoriasis, in addition to this PSOR1 gene, several kinds of genes localized in chromosome 1, chromosome 3, chromosome 4, chromosome 16, chromosome 17, chromosome 19, and chromosome 20, have been reported to date.
However, there are many points in the mechanisms of onset of psoriasis which have not yet been clarified, and the development of a therapeutic method or the like is difficult. In order to connect to the development of a new diagnostic or therapeutic method, advancements in research and analysis of a gene involved in the onset of psoriasis (psoriasis-associated gene) has been desired.
Pathologies of psoriasis and the like are summarized in some reviews (Bos et al., Immunol. Today, 20, 40-46, 1999 ; Baker et al., Clin. Exp. Dermatol., 26, 321-325, 2001).
On the other hand, the phospholipase A2 has been known as an enzyme involved in inflammatory diseases. The phospholipase A2 is an enzyme which catalyzes a reaction in which an ester bond at 2-position of a glycerophospholipid is hydrolyzed to generate equimolar amounts of a free fatty acid and a lysophospholipid. Arachidonic acid released from glycerophospholipid constituting a biomembrane by action of phospholipase A2 is converted into leukotrienes by lipoxygenase or the like, or converted into prostaglandins with cyclooxygenase or the like, or further converted into thromboxane A2 or the like by action of thromboxane synthase on the prostaglandins. Since it has been revealed that these eicosanoids (leukotrienes, prostaglandins, and thromboxanes) are closely involved with various inflammations, allergic reactions, and ischemic diseases, the phospholipase A2 plays a key role as an enzyme which raises a primary reaction that results in production of inflammatory mediators downstream of arachidonic acid.
It has been known that the phospholipase A2 has at least 17 kinds of molecular species. These molecular species are classified into four subclasses (secretary phospholipase A2, cytoplasmic phospholipase A2, Ca2+-independent phospholipase A2, and platelet activating factor acetylhydrolase), based on similarities in structure and characteristics.
Among them, as cytoplasmic phospholipase A2 (cPLA2), which is high-molecular weight phospholipase A2 localized in a cytoplasm, three molecular species [cPLA2α (GenBank Ac. No. M72393; Clark et al., Cell, 65, 1043-1051, 1991), cPLA2β (GenBank Ac. No. AF065215; Pickard et al., The Journal of Biological Chemistry, 274, 8823-8831, 1999), cPLA2γ (GenBank Ac. No. AF058921; Underwood et al., The Journal of Biological Chemistry, 273, 21926-21932, 1998)] have been so far identified. cPLA2γ exhibits activity in a Ca2+ concentration-independent manner. On the other hand, cPLA2α and cPLA2β each has a phospholipid-binding region at N-terminal side of a protein molecule, and exhibits its activity by binding to a phospholipid membrane in a Ca2+ concentration-dependent manner. However, since the amount of Ca2+ required for exhibiting the activity is on the order of μM, it is considered that these also exhibit their activities in cytoplasm.
The phospholipase A2 is an important target molecule in the development and research of therapeutic drugs for inflammatory diseases, and studies of inhibitors thereof have been advanced intensively. There are some cases where inhibitory action for the phospholipase A2 has been found in known medicaments. In addition, it has been also found that a specific phospholipase A2 inhibitor can serve as a useful therapeutic drug.
In order to develop an excellent medicament having high therapeutic effects and little side effects, it has been desired to screen an inhibitor having a high selectivity on a particular type of the phospholipase A2 to be targeted.
Further, it has been also desired to find a novel type of the phospholipase A2, which is a different molecular species from those of conventional ones, for the studies of the mechanism of onset of inflammatory diseases, and for the possibility to be used as a target molecule of a novel therapeutic drug.